1. Field of the Invention
The present invention relates to the field of marine pipelaying, wherein a pipeline is laid from an offshore vessel onto the seabed.
2. Description of Background Art
In this field is it known to use a pipeline supporting stinger to support the pipeline from the vessel into the water. The vessel can be of any suitable design, including a monohull vessel, a semi-submersible, a barge, etc. Often said stinger provides a curved or semi-curved path for the pipeline, e.g. when the pipeline is assembled or paid out in a generally horizontal direction from the vessel. The design of the stinger commonly depends on factors like the envisaged water depth, pipeline size and weight, pipeline properties (stiffness, material(s), possible coating, etc), pipeline tension during laying, etc.
For some years pipelaying has been done or at least proposed for deepwater, which has led to design of very large stingers which allow supporting the pipeline along a significant arc, up to an essentially 90 degrees arc, from horizontal. Stingers have been designed having a weight of more than 100 hundred tonnes, even several hundreds of tonnes. It is noted that the pipeline suspended between the stinger and the seabed also has a very significant weight, which load is carried by said stinger.
In WO 2005/123556 an example is shown of a pipelaying vessel having a stinger and an associated stinger suspension assembly. In said example the stinger suspension assembly is connected at one end to the foot of a very large crane on stern of the vessel. Several stinger suspension assemblies are proposed therein which are adapted to raise and lower the stinger.
In another known design it is known to arrange two cantilevers on the stern of the vessel, which project rearward above the water and each carry a winch with a cable from which the stinger is suspended.
Many prior art vessels having a stinger are designed for typical S-lay, wherein the vessel is provided with a pipeline-laying installation with one or more connecting stations, usually welding stations, for connecting pipe sections in a substantially horizontal orientation on a deck of the vessel. It is noted that stingers are also known on vessels for laying reeled pipeline, which pipeline to be laid is stored on one or more reels.
In the field of pipelaying the continuity of the pipelaying process is a prime factor. The weather and associated wave conditions generally form a limitation on the continuity as pipelaying has to be suspended under certain unfavourable conditions in order not to compromise the pipeline and/or the pipeline installation system.